The present invention relates, in general, to condenser microphones, and more particularly, to a condenser microphone formed with semiconductor processing techniques.
In the past, a variety of techniques were utilized to form condenser or capacitive microphones on silicon substrates. Typically, a cavity is etched into a silicon substrate in order to form a thin single crystal silicon diaphragm that functions as one plate of a capacitor. The second capacitor plate typically is formed as a metal layer overlying the diaphragm. Because the diaphragm is formed by etching a cavity in a silicon substrate, it is difficult to control the thickness of the diaphragm therefore difficult to control the diaphragm stiffness and the resulting sensitivity of the microphone.
Often the spacing between the plates of the capacitor generally are established by the thickness of a photoresist layer. Because photoresist can not be exposed to high (above 150 degrees Celsius) temperatures, standard semiconductor deposition techniques can not be used for the fixed plate. Additionally, the fixed capacitor plate often is formed either by electroplating a metal overlying the diaphragm, or by wafer bonding techniques. Electroplating is not compatible with standard semiconductor processing techniques because electroplating can result in intermetallic diffusions and other contaminants that can not be integrated into the manufacturing flow thereby increasing microphone costs. Because it is difficult to control diaphragm thicknesses, stress, and plate spacing resulting from wafer bonding, it is also difficult to control the sensitivity of microphones utilizing wafer bonding techniques.
Additionally, many of the prior art techniques utilized temperatures that were in excess of what can be tolerated by semiconductor circuits or else resulted in forming active semiconductor devices spaced laterally from the capacitor thereby resulting in a large semiconductor die size and increased costs.
Accordingly, it is desirable to have a capacitive microphone and method therefor that results in an easily manufactured and low cost capacitive microphone, that has well controlled sensitivity, that has well controlled diaphragm stiffness, that has well controlled diaphragm capacitor plate spacing, that includes integrated active semiconductor devices, and that has a small die size.